There have been used an inverter unit for obtaining an AC voltage having a constant frequency (generally a commercial frequency) by using a generator driven by a prime mover such as an internal combustion engine as a power source. As shown in FIG. 6, an inverter unit for converting an output of an AC generator into an AC output having a constant frequency includes: an AC/DC converter CNV for converting an output of an AC generator into a DC output; an inverter INV for converting the output of the converter into an AC voltage of a constant frequency; and a filter FL. The filter FL is a low-pass filter having, for example, first and second coils L1, L2 and a capacitor C1, and is provided for removing a high harmonic wave component from the output of the inverter INV.
FIG. 7 is a schematic diagram showing an example of a structure of a conventional inverter unit 401 of this type. In FIG. 7, a reference numeral 402 shows a box-like case having a rectangular shaped bottom wall portion 402a, and a side wall portion comprising side walls 402b, 402c facing to the longitudinal direction of the bottom wall portion 402a and side walls 402d, 402e facing to the cross direction of the bottom wall portion 402a. In the case 402, there are housed component parts of the inverter unit, including the converter CNV, the inverter INV and the filter FL.
The first and second coils L1, L2 of the filter FL are wound around first and second bobbins B1, B2 mounted to first and second cores I1, I2, respectively. In order to maintain the stability of cut-off frequency of the filter with respect to the change of load current, a prismatic core being comprised of a laminated steel plate and extending straight is used as the cores I1, I2 being wound with the coils L1, L2, respectively.
The coils L1, L2 are arranged at a position close to the side wall portion 402b which is one end of the case 402 so that central axes of the coils coincide with each other, and an end portion I1b of the core I1 of the first coil L1 and an end portion (not shown) of the core I2 of the second coil L2 are respectively arranged close to the side walls 402d, 402e facing to the cross direction of the case 402. The capacitor C1 and the component parts of the converter CNV and the inverter INV are mounted to a printed board 403 which is arranged by the coils L1, L2.
It may be possible to arrange the coils L1, L2 at the central of the case 402. However, in this case, the structure of the unit becomes complicated, an outlet of an output line from the filter becomes complex, and the cost increases because of an increase in manufacturing processes, since it becomes necessary to divide the printed board 403 into two parts.
On the other hand, if the coils L1, L2 are arranged on the end portion of the case 402 as shown, it is possible to simplify the structure of the unit since it is unnecessary to divide the printed board 403. In addition, it is possible to make the outlet of the output line from the filter FL to outside easily in the case where the coils L1, L2 are arranged on the end portion of the case 402.
The inverter unit of this type often provides a function as a heat sink to the case 402 in order to improve heat radiation from the converter and the inverter. Also, for preventing the weight of the inverter becoming heavy, it is desirable to use a case as light as possible. Therefore, the case 402 is generally made from aluminum which not only has good heat conductivity but also is light weight.
Although it is not shown in the drawings, resin is cast into the case 402, and at least a part of the component parts of the unit is molded by the resin, so that the inverter unit has water-resisting and earthquake-resisting characteristics. An inverter unit of this type is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 11-122932.
An inverter unit shown in FIG. 7 includes an AC/DC converter CNV for converting an AC output of a generator into a DC output because it is assumed that an AC generator is used as power source. However, the AC/DC converter may be omitted in the case where an output of a DC power source such as a storage battery and a solar battery is converted into an AC output of constant frequency.
As shown in FIG. 7, if the coils L1, L2 are disposed at the end portion of the case 402 made of aluminum, and the end portions of the cores I1, I2 being wound with the coils L1, L2, respectively, are arranged closed to the inner surface of the side walls 402d, 402e of the case 402, the magnetic flux coming in and out the end portions of the cores I1, I2 flows through the side walls 402d, 402e of the case. If an AC magnetic flux flows through the side walls 402d, 402d of the case, eddy current flows at the side walls 402d, 402e. Then, since magnetic flux generated by the eddy current prevents an AC magnetic flux flowing through the cores I1, I2 from changing, magnetic resistance of the cores I1, I2 is increased, and inductance of the coils L1, L2 is decreased.
Therefore, it has been necessary for the conventional inverter unit of this type to make the coils L1, L2 have larger inductance than their needs, with making allowance for its possible reduction of inductance of the coils L1, L2. Thus, it was inevitable for the coils L1, L2 to be large and the cost to be expensive.
To avoid the reduction of inductance of the coils L1, L2, the distance between the end portions of the cores I1, I2 and the inner surfaces of the side walls 402d, 402e of the case 402 may be enlarged. However, in this case, the case 402 becomes large, which causes the inverter unit to become large.